Analog-to-digital converters have seen increased use in recent years due to the advances in digital signal processing and the increased use of digital transmission systems. Typically, analog-to-digital converters comprise circuitry for receiving an analog input signal and outputting a digital value that is proportional to the analog input signal. This digital output value can either be a parallel word or a serial digital bit string. There are many types of analog-to-digital conversion schemes such as voltage-to-frequency converters, charge redistribution, delta modulation, etc. Each of these techniques has advantages and disadvantages associated therewith.
One type of analog-to digital converter that has seen increased use in recent years is that utilizing delta-sigma modulation wherein an analog voltage is input to a delta-sigma modulator and the output thereof filtered to remove noise. The delta-sigma modulator is of the type which converts an analog input to a digital pulse string having an average amplitude over time proportional to the analog input. One type of delta-sigma pulse modulator is described in U.S. Pat. No. 4,542,354, issued Sept. 17, 1985 to Robinson, et al. Delta-sigma modulation provides for high accuracy and wide dynamic range as compared to earlier delta modulation techniques. The delta-sigma type modulation is sometimes referred to as an oversampled converter architecture which is immune from some of the earlier undesirable second order effects of delta modulation.
There are two key components of a delta-sigma analog-to-digital converter, the analog modulator and the digital filter. The analog modulator oversamples the analog input and produces a low resolution digital output. However, with any analog-to-digital converter, there are a number of noise sources that are inherent to any analog modulator design. In a delta-sigma modulator, there are output stage noise sources and input stage noise sources, the output noise sources normally being dominated by quantization noise and the input noise sources resulting from DC offset and 1/f noise. The quantization noise at low frequencies is relatively low with the largest portion thereof existing at higher frequencies. This higher frequency portion of the noise can be filtered out by a digital domain low-pass filter. However, the low frequency DC offset and 1/f noise cannot be filtered out by a low-pass filter, and, as such, this noise will be passed through the filter with the signal information.
One approach to minimizing the low-frequency noise in an analog-to-digital converter utilizing a delta-sigma modulator is chopper stabilization, described in pending application, U.S. Pat. Ser. No. 205,996, filed June 13, 1988 by Adrian Early and assigned to the present Assignee, which application is incorporated herein by reference. In the pending application, the input integrator to the analog modulator is chopper stabilized at F.sub.S /2. Although the chopper stabilization itself acts to minimize the low frequency noise, there still exists the possibility for the high frequency noise which exists at the F.sub.S /2 and F.sub.S harmonics to be modulated downward into the baseband of the modulator. With the chopper stabilization of the pending application at F.sub.S /2, the high frequency noise is filtered out at the output of the modulator with the post digital filter. However, internal to the modulator, the high frequency noise still exists. Since the chopping frequency is present, this can result in noise being modulated down from high frequencies into the baseband.
For DC inputs, the delta-sigma modulator output will be a series of 0's or 1's that will be repetitive at some integer multiple of the output word rate. The frequency response of this output bit stream will show natural tones which will rise above the quantization noise floor in the region of F.sub.s /4 to F.sub.s /2. Chopping can modulate these tones and the quantization noise of the converter at F.sub.s /2 into the baseband. This will cause an increase in baseband noise and consequently, a decrease in dynamic range of the converter. Unfortunately, quantization noise in delta-sigma converters is usually at its maximum value at F.sub.s /2.